The present invention relates to a method of welding together aluminum alloy workpieces of the same aluminum alloy. The welding is carried out by depositing a selected alloying metal at the workpiece interfacing the weld area and locally heating the workpiece in the weld area forming a localized liquid solution and subsequent solidified high strength weld seam. The localized heating of the workpiece can be carried out using a laser or an electron beam or other such localized heating means.
In the manufacture of electrical systems, such as radar systems, numerous electronic packages must be hermetically sealed. These electronic packages are typically box-like with radio frequency connections sealed through the package or module walls. Electronic components such as hybrid microwave circuitry are mounted within the package which is hermetically sealed by soldering in place a top or bottom cover member. It has been common to solder-seal such electronic packages but this requires heating the entire package, which is typically aluminum or a selected aluminum alloy, to a temperature which may adversely affect the electronic components within the package.
It is also not unusual for such electronic packages to be reopened or delidded during assembly and testing for adjustment or replacement of defective electronic components. In such a repair or rework operation, the package must be disassembled by removal of the cover which requires reheating of the solder-seal to permit removal of the cover member. Following replacement or repair of the electronic component, the cover then would be solder-sealed back onto the package to complete assembly.
The laser welding of aluminum workpieces or packages has proven to be a highly desirable fabricating process for many applications, such as in airborne electronic subsystem structures or packages containing electronic components, wherein hermetic sealing is required for hybrid microelectronics, radio frequency signal processing modules, power hybrid packages, and other similar electronics packages. The primary advantages of laser welding are the relative ease of automation of the process with the attendant lower cost of fabrication, and the minimization of heat transferred to the electronic components during the laser welding process. Appropriate design of the weld joint configuration also allows repetitive opening and rewelding of the module lid or cover to provide for rework of the electronic components contained within the package.
The fabrication of phase array radar antenna structures and microwave waveguide manifolds is particularly difficult using conventional welding techniques. Such structures involve very tight tolerances of machining dimensions for the pieces which are to be joined, and this joining must be effected with minimal thermal distortion. The welding process of the present invention with its highly localized heating of the weld joint, and use of the wave selected aluminum alloy is particularly advantageous for fabricating such radar antenna systems.
The preferred aluminum package material from a structural standpoint for the housing and covers of an electronic package is 6061 aluminum alloy which has many highly desirable structural properties and is very easily machined for fabricating ease. This alloy which contains 0.6% silicon, 1% magnesium, and 0.3% copper, however, shows a marked tendency to crack in the weld area where the joint components or workpieces are both 6061 aluminum alloy. It has been the practice in joining 6061 aluminum alloy workpieces together to utilize a filler material such as 4047 aluminum alloy which is typically available as welding rod or shim stock, or to use the 4047 alloy as a cover member to effectuate the hermetic sealing of the 6061 alloy component. The 4047 aluminum alloy contains about 12% silicon and when the 4047 alloy is locally heated to form a weld, it results in a nominal silicon concentration of typically about 2 to 5% in the weld area which is known to be a high strength non-cracking weld. The 4047 aluminum alloy has a flexural modulus and hardness characteristic which are significantly below the desired characteristics exhibited by 6061 aluminum alloy.
U.S. Pat. No. 4,521,668, owned by the assignee of the present invention, teaches laser welding a 4047 aluminum alloy cover member to a 6061 aluminum alloy electronics package to effectuate hermetic sealing of the aluminum electronics package. This patent teaches that the laser sealing fabrication process minimizes heating of the electronics components within the package during sealing and possible subsequent delidding and resealing. The teachings of this aforementioned patent are limited in that in order to effectuate high quanlity, high structural strength welds it was necesary to employ the 4047 aluminum alloy. It is highly desirable that the aluminum package be entirely fabricated of 6061 aluminum alloy both for the housing of the electronic package as well as for the cover member. It is, therefore, highly desirable that a method of welding together 6061 aluminum alloy workpieces be found.
A method of laser welding various metals while depositing a material which improves the absorption of the infrared laser radiation is taught by Japanese Patent Application No. 58-65592. The teaching with respect to the welding of aluminum is to utilize iron or nickel as the infrared radiation absorbing material in the weld zone between the workpieces. In another laser welding process taught by Japanese Patent Application No. 59-30492, an aluminum alloy filler is placed between an aluminum base material and an aluminum alloy workpiece. The aluminum alloy filler member between the workpieces differs from the welding base material aluminum alloy. The filler aluminum alloy melts under localized heating as by laser heating and the high temperature strength of the weld is improved by the silicon contained within the filler alloy.
U.S. Pat. No. 4,251,709 teaches a welding process using a localized heating source such as an electron beam welder or a laser, with a jet of liquid metal directed onto the seam or weld area between the workpieces which are welded together. The localized heating means is used to melt the workpiece surface, and subsequent solidification of the liquid metal deposited in the weld area produces a solid metal joint between the workpieces.